Reproduction in Paramaecium may be effected non-sexually, by fission, the body dividing transversely into two halves, and the process of cleavage commencing first with the division of the nucleus (fig. 32, B). Longitudinal fission is also stated to occur (fig. 32, C); but it is questionable whether the appearances which have led to this statement may not really be due to the coalescence and temporary conjugation of two individuals. Most authorities further believe that Paramaecium has the power of true sexual reproduction, the "nucleus" playing the part of an ovary in the process, and the "nucleolus " acting as a testis. In this process, as described by Bal-biani, two Paramaetria come together, and adhere closely to one another by their ventral surfaces. The "nucleus" increases in size, and a number of ovules are formed in its interior. In like manner, the "nucleolus" of each also enlarges, and develops in its interior a number of fusiform or rod-like bodies, which are believed to be spermatozoa. The nucleolus of each then passes into the body of the other, the act of transference being effected through the mouth. Contact of the two reproductive elements then takes place, and the fecundated ovules after their liberation from the body of the parent are developed into adult Paramaecia.
Other observers, however, are disposed to believe that this "conjugation" of two Paramaecia is not a genuine sexual process, that the "nucleolus" is not a true testis, and that the rounded bodies into which the "nucleus" breaks up can be developed into new individuals directly and without contact with a second reproductive element.
Vorticella (fig. 33, C) is a beautiful flower-like Infusorian which is commonly found in fresh water, adhering to the stems of aquatic plants. It consists of a bell-shaped body or "calyx," supported upon the extremity of a slender contractile stem or "pedicle." The other extremity of the pedicle is fixed to some foreign body, and its power of contraction is due to the presence in its interior of a spiral contractile fibre, which is sometimes called the "stem-muscle" The edge of the bell or calyx is surrounded by a projecting rim or border, called the "peristome," within which is a circular surface, the "disc," forming the upper extremity of the so-called "rotatory organ." The disc is surrounded by a fringe of vibratile cilia, forming a spiral line which is prolonged into the commencement of the digestive canal. Near the edge of the disc is situated the mouth, which conducts by its entrance or "vestibulum" into a fusiform canal or "pharynx," which terminates abruptly in the abdominal cavity. The particles of food are taken in at the mouth, descend through the short alimentary canal, and enter the abdominal cavity, where they are subjected to the general rotation of the "chyme-mass," being finally excreted by an anal aperture which is situated near the mouth and within the vestibule. As in Paramaecrium, the body in Vorticella is composed of an outer "cuticle," a central "chyme-mass," and an intermediate "cortical layer," which contains a contractile vesicle and a band-like nucleus.
Reproduction in Vorticella may take place by fission, or by gemmation, or by a process of encystation and endogenous division. In the first of these modes the calyx becomes indented in a longitudinal direction - viz., from the pedicle to the disc ; and the groove thus formed becomes gradually deeper until the calyx is finally divided into two halves supported upon the same pedicle. On one of these cups a "posterior" circlet of cilia is then formed in addition to the "anterior" circlet already existing (i.e., a fringe of cilia is developed round that end of the calyx which is nearest the attachment of the pedicle and furthest from the disc). The cup (fig. 33, D), thus furnished with a circlet of cilia at both extremities, is then detached, and swims about freely. Finally, the anterior circlet of cilia disappears, and this end of the calyx puts forth a pedicle and becomes attached to some foreign object. A new mouth is now formed within what was before the posterior circlet of cilia; so that the position and function of the two extremities of the calyx are thus reversed.
In the second mode of reproduction - namely, that by gemmation -exactly the same phenomena take place, with this single difference, that in this case the new individual is not produced by a splitting into two of the adult calyx, but by means of a bud thrown out from near its proximal extremity. This bud is composed of a prolongation of the cuticular and cortical layers of the adult with a caecal diverticulum of the abdominal cavity or chyme-mass. It soon develops a posterior circlet of cilia, the connection with the parent is rapidly constricted until complete separation is effected, and then the process differs in no respect from that described as occurring in the fissiparous method of reproduction. According to Stein and Greeff, however, these so-called "buds" are really small calyces, produced by fission of one Vorticella and then attaching themselves to the outside of the calyx of an6ther individual.
In the third mode of reproduction the Vorticella encysts itself in a capsule, the cilia and pedicle disappear, and the nucleus breaks up into a number of rounded germs, which are ultimately liberated by the rupture of the cyst, and after a short locomotive stage, develop themselves into fresh Vorticellae. How far this process may be truly sexual is not known, and no form of unequivocal sexual reproduction has hitherto been shown to occur in the case of Vorticella.
Epistylis is a not uncommon form of fixed Infusorian which is nearly allied to Vorticella, and differs chiefly in the fact that the pedicle is much branched, and rigid and not contractile. It usually occurs in the form of a greyish-white nap on the stems of water-plants, or on the head of the common water-beetle, the Dytiscus marginalis. It consists of a plant-like branching and re-branching frond, the stems of which are quite transparent and faintly striated, but are not contractile, though capable of movement from side to side. Each branch of the entire colony terminates in an oval calyx, articulated to the stem by a distinct joint, upon which it can move from side to side. The sarcode-body enclosed within the cortical layer is of a light-brown colour, and full of minute granules, with larger food-vacuoles and a well-marked contractile vesicle, which contracts and dilates two or three times a minute. The animal can retract itself entirely within its cup, and can at will exsert a ciliated disc.
Carchesium is another form which is like Epistylis in consisting of a number of calyces supported upon a branched pedicle, but differs from Epistylis and agrees with Vorticella in the fact that the pedicle is contractile.
Stentor, or the trumpet-animalcule (fig. 33, A), is another common Infusorian which is closely related to Vorticella. It consists of a trumpet-shaped calyx, devoid of a pedicle, but possessing the power of attaching and detaching itself at will. When detached it swims by means of the anterior circlet of cilia, just as the calyx of Vorticella will if broken from its stalk. In Vaginicola (fig. 33, B) the essential structure is much the same as in Vorticella, but the body is protected by a membranous or horny case ("carapace" or "lorica"), which is formed by a hardening of the cuticle, and within which the animal can retire.